The ErbB/HER subfamily of polypeptide growth factor receptors include the epidermal growth factor (EGF) receptor (EGFR/ErbB1/HER1), the neu oncogene product (ErbB2/HER2), and the more recently identified ErbB3/HER3 and ErbB4/HER4 receptor proteins (see, e.g., Hynes et. al. (1994) Biochim. Biophys. Acta Rev. Cancer 1198, 165-184). Each of these receptors is predicted to consist of an ectodomain (extracellular ligand-binding domain), a membrane-spanning domain, a cytosolic, protein tyrosine kinase (PTK) domain and a C-terminal phosphorylation domain (see, e.g., Kim et al., (1998) Biochem. J. 334, 189-195). The ectodomains of the ErbB receptors are further characterized as being divided into four domains (I-IV). Domains I and III of the ErbB ectodomain are involved in ligand binding (see, e.g., Hynes et. al. (2005) Nature Rev. Cancer 5, 341-354). Ligands for these receptors include heregulin (HRG) and betacellulin (BTC).
Experiments in vitro have indicated that the protein tyrosine kinase activity of the ErbB3 receptor (ErbB3) protein is attenuated significantly relative to that of other ErbB/HER family members and this attenuation has been attributed, in part, to the occurrence of non-conservative amino acid substitutions in the predicted intracellular catalytic domain of ErbB3 (see, e.g., Guy et al. (1994) Proc. Natl. Acad. Sci. USA. 91, 8132-8136; Sierke et al. (1997) Biochem. J. 322, 757-763). However, the ErbB3 protein has been shown to be phosphorylated in a variety of cellular contexts. For example, ErbB3 is constitutively phosphorylated on tyrosine residues in a subset of human breast cancer cell lines overexpressing this protein (see, e.g., Kraus et al. (1993) Proc. Natl. Acad. Sci. USA. 90, 2900-2904; and Kim et al. Supra; see, also, Schaefer et al. (2006) Neoplasia 8(7):613-22 and Schaefer et al. Cancer Res (2004) 64(10):3395-405).
Although, the role of ErbB3 in cancer has been explored (see, e.g., Horst et al. (2005) 115, 519-527; Xue et al. (2006) Cancer Res. 66, 1418-1426), ErbB3 remains largely unappreciated as a target for clinical intervention. Current immunotherapies primarily focus on inhibiting the action of ErbB2 and, in particular, heterodimerization of ErbB2/ErbB3 complexes (see, e.g., Sliwkowski et al. (1994) J. Biol. Chem. 269(20):14661-14665 (1994)). Accordingly, it is an object of the present invention to provide improved immunotherapies that effectively inhibit ErbB3 signaling, and can be used to treat and diagnose a variety of cancers.